Generate a JSON value

You need to identify the name elements as being within the array by preceding the field name with the array name and dot(.):

• name.first_name
• name.last_name
• name.full_name

then set full name as a 'Template' field type and create the combined string:
{last_name}, {first_name}
See example: https://www.mockaroo.com/2502ed30

If they must be in the order you have in the question, then you need to first create the first name and last name as hidden fields (precede with two underscores (__):

• name.__first_name
• name.__last_name

Then return all three fields as template fields.